1. Field of the Invention
The present invention relates to a nitride semiconductor device and a manufacturing method thereof, and particularly to a nitride semiconductor device having an n-electrode provided over the back surface of a nitride semiconductor substrate.
2. Description of the Background Art
As devices like blue light-emitting diodes (LEDs) and blue laser diodes (LDs) are put in practical use, nitride semiconductor devices using, e.g., gallium nitride (GaN), indium gallium nitride (InGaN), and aluminum gallium nitride (AlGaN), are attracting increased attention.
Usually, the crystals of nitride semiconductors, such as GaN, have hexagonal wurtzite structure. Accordingly, a nitride semiconductor substrate formed through crystal growth has a polarity, and so has a front surface called Ga surface and a back surface called N surface.
For the crystal growth of nitride semiconductors, it is known that the growth on the front surface (Ga surface) of a nitride semiconductor substrate offers good crystal quality. Accordingly, in the manufacture of nitride semiconductor devices, the layered-structure of the semiconductor element is usually formed by crystal growth of nitrides over the front surface of the nitride semiconductor substrate. Accordingly, in a laser diode as a conventional nitride semiconductor device, both of its n-electrode and p-electrode are formed over the front surface of the substrate. However, the manufacturing process then requires removing part of the layered-structure of the semiconductor element in order to expose the surface of the substrate, and this complicates the manufacturing process. Also, providing two electrodes on one side (on the front surface side) of the substrate requires a device formation area that is about two times larger than the area required when one electrode is formed on each side, which hinders miniaturization of the semiconductor device.
Accordingly, nitride semiconductor devices having electrodes on both sides are recently developed in which the n-electrode is formed over the back surface of the nitride semiconductor substrate (for example, see Japanese Patent Application Laid-Open Nos. 2004-71657 and 11-340571 (1999), which are hereinafter referred to respectively as Patent Document 1 and Patent Document 2).
Patent Document 2 discloses a technique in which, during a process of fabricating a laser diode, a GaN layer (n-contact layer) doped with silicon is grown on the back surface of a GaN substrate, and the n-type electrode is formed on the GaN layer.
On the back surface (N surface) of the nitride semiconductor substrate, deposited metal is more likely to exfoliate than on the front surface (Ga surface). Accordingly, a metal electrode formed on the back surface of the substrate tends to exhibit larger resistance (contact resistance) between the electrode and the substrate. For example, in a laser diode, if the contact resistance of the n-electrode is not sufficiently low, an increased voltage (operating voltage) is required to cause the laser diode to operate, and electric characteristics vary due to the generation of heat during operation. This makes it difficult to obtain stable output in the prescribed temperature range. It is therefore desirable to further reduce the contact resistance of the n-electrode provided over the back surface of the nitride semiconductor substrate.
When, as described in Patent Document 2, a silicon-doped GaN layer is grown on the back surface of a GaN substrate and an n-electrode is formed on the GaN layer, the n-electrode offers enhanced ohmic properties and enhanced adhesion as compared with conventional ones. However, when a thermal treatment is performed after the formation of the n-electrode, the reaction between the GaN layer surface and the n-electrode progresses and then the electric characteristics at the interface between the GaN layer and the n-electrode may be deteriorated, depending on the conditions of the thermal treatment (for example, the barrier height is increased (the tunneling is reduced)).